One of conventionally used transmitting methods for mobile radio communication systems is the orthogonal frequency division multiplexing (OFDM) scheme. The OFDM scheme is used in communication systems, for example, Long Term Evolution (LTE, or Super 3G) and Worldwide Interoperability for Microwave Access (WiMAX). According to the OFDM scheme, a modulated signal is mapped onto the frequency domain and is then converted into a time-domain signal. Fast Fourier transform (FFT) is used to transform a signal between the time domain and the frequency domain. For example, there are receiving apparatuses for performing fast Fourier transform on signals received through a plurality of receiving antennas to transform the received signals for respective subcarriers. One of such receiving apparatuses is an apparatus for detecting a delay-time portion greater than a guard interval of data as an intersymbol interference (ISI) portion, generating a time-waveform portion of a known symbol corresponding to the ISI portion as a replica, subtracting the replica from a received signal, and performing FFT on the result of subtraction to demodulate the data (see, for example, disclosed in Japanese Laid-open Patent Publication No. 2004-208254). Another example of the receiving apparatuses is a receiving apparatus for generating a replica of mutual interference components between systems for each subcarrier, subtracting the replica from each of received signals of the systems subjected to fast Fourier transform to cancel the mutual interference components between the systems included in the received signal (see, for example, disclosed in Japanese Laid-open Patent Publication No. 2004-235916).
In such a conventional receiving apparatus, when received signals are subjected to fast Fourier transform, a difference in average power between subcarriers subjected to the transform occurs in some cases. For example, when the received power of subcarriers allocated to a distant terminal is higher than that of subcarriers allocated to a nearby terminal, the difference in average power between subcarriers subjected to fast Fourier transform may occur. In this case, an overflow occurs in a subcarrier having high power after fast Fourier transform unless a sufficient number of bits is used in fast Fourier transform or a high level diagram is designed. If an overflow occurs, an error may be reduced by saturation. Accordingly, saturation is usually performed in operations for radio communication.
As for the subcarrier having high power, the level of an input signal to be subjected to fast Fourier transform is controlled so that the occurrence of overflow may be prevented. In this case, the number of allocated bits may be reduced in the subcarrier having low power, so that the accuracy of fast Fourier transform is lowered. To yield sufficient accuracy on the subcarrier having low power, the number of bits necessary for fast Fourier transform has to be increased. The code division multiple access (CDMA) scheme, one of the transmitting methods for the mobile radio communication systems, uses fast Hadamard transform (FHT) instead of fast Fourier transform. Fast Hadamard transform has the same problem as that described above.